$::foo always refers to $main::foo, while $foo is the package variable $Namespace::foo for whatever your current namespace is. They may be identical, but not always.

Perl always sees a variable name containing '::' as a fully qualified name.

Even without a lexical $foo, and in package main, $foo may differ from $::foo:

$ perl -we'package Foo; our $foo = 1; package main; $main::foo = 2;
print "\$foo: $foo \$::foo $::foo\n"'
$foo: 1 $::foo 2
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C:\>perl -MO=Deparse -e "$foo = 'bar'";
$foo = 'bar';
-e syntax OK

C:\>perl -MO=Deparse -e "$main::foo = 'bar'";
$foo = 'bar';
-e syntax OK

C:\>perl -MO=Deparse -e "$::foo = 'bar'";
$foo = 'bar';
-e syntax OK
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Update

Superficially these all parse the same. But as Corion notes the :: is significant as it signifies a package rather than a lexical variable. As soon as your current package is not main but some other package, or as soon as $foo is a lexical variable (and $::foo will never be) they are different.

C:\>perl -MO=Deparse -e "package Foo; my $foo = 'bar'";
package Foo;
my $foo = 'bar';
-e syntax OK

C:\>perl -MO=Deparse -e "package Foo; my $::foo = 'bar'";
"my" variable $::foo can't be in a package at -e line 1, near "my $::f
+oo "
Can't use global $::foo in "my" at -e line 1, near "my $::foo "
-e had compilation errors.

C:\>perl -e "package Foo; $::foo = 'foo'; print $foo"   


C:\>perl -e "package Foo; $::foo = 'foo'; print $main::foo"
foo
C:\>
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